Abstract
Migratory birds can detect the direction of the Earth’s magnetic field using the magnetic compass sense. However, the sensory basis of the magnetic compass still remains a puzzle. A large body of indirect evidence suggests that magnetic compass in birds is localized in the retina. To confirm this point, an evidence of visual signals modulation by magnetic field (MF) should be obtained. In a previous study we showed that MF inclination impacts the amplitude of ex vivo electroretinogram (ERG) recorded from isolated pigeon retina. Here we present the results of an analysis of putative MF effect on one component of ERG, the photoreceptor’s response, isolated from the total ERG by adding sodium aspartate and barium chloride to the perfusion solution. Photoresponses were recorded from isolated retinae of domestic pigeons Columba livia. The retinal samples were placed in MF that was modulated by three pairs of orthogonal Helmholtz coils. Light stimuli (blue and red) were applied under two inclinations of MF, 0° and 90°. In all the experiments, preparations from two parts of retina were used, red field (with dominant red-sensitive cones) and yellow field (with relatively uniform distribution of cone color types). In contrast to the whole retinal ERG, we did not observe any effect of MF inclination on either amplitude or kinetics of pharmacologically isolated photoreceptor responses to blue or red half-saturating flashes. A possible explanations of these results could be that magnetic compass sense is localized in retinal cells other than photoreceptors, or that photoreceptors do participate in magnetoreception, but require some processing of compass information in other retinal layers, so that only whole retina signal can reflect the response to changing MF.
Highlights
Migratory birds use the Earth’s magnetic field (MF) for navigation and orientation during their long-distance journeys between breeding and wintering areas, and are known to possess both a magnetic compass [1] and a magnetic positioning system, a map [2]
A large body of behavioral data indicates that the avian magnetic compass has several important properties: it is based on the inclination angle of the magnetic field lines rather than on the polarity of the field [1], it is light-dependent, and, it depends on the light spectral composition
We found no significant effects of MF inclination change on the amplitude of photoreceptor responses to blue (n = 16, t = -1.045, p = 0.312) or red flashes (n = 16, t = -0.365, p = 0.720; see Fig 5A)
Summary
Migratory birds use the Earth’s magnetic field (MF) for navigation and orientation during their long-distance journeys between breeding and wintering areas, and are known to possess both a magnetic compass [1] and a magnetic positioning system, a map [2]. Searching for magnetic compass mechanism in pigeon retinal photoreceptors light and disoriented under yellow and red light [3–8, for a review see 9] Based on this fact, the primary magnetoreceptor in birds is supposed to be localized in the retina of the eye, and the prevalent hypothesis describing the work of such a magnetoreceptor is the radical pair model [10, 11; reviewed in 12]. The primary magnetoreceptor in birds is supposed to be localized in the retina of the eye, and the prevalent hypothesis describing the work of such a magnetoreceptor is the radical pair model [10, 11; reviewed in 12] According to this model, the primary receptor molecules that perceive the magnetic field are the cryptochromes. At present, essentially nothing is known about the molecular mechanisms that might enable cryptochromes to transduce neuronal signals to the brain [for a review see 14]
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